1. Field of the Invention
The invention relates to an apparatus for leveling wet concrete that exhibits the ability to detect, indicate and adjust its own deflection through the use of a laser light beam and electronic sensing device.
2. Description of Related Art
In the construction industry a long straight beam called a straight-edge is often used during the leveling of wet concrete. In its most primitive form, a long straight piece of wood is used to cut down high and fill in low spots or "strike off" the uneven concrete. This process is called screeding.
Screeding distributes and places the concrete but does little in providing a quality finished surface. Another tool, often called a "bull float", is then used to produce the desired smooth quality surface texture. Because it is shorter than a straight-edge, use of the bull float disturbs the flatness of the concrete after screeding, creating new high and low areas despite the smoother surface texture.
For a number of reasons, simply using a longer bull float proves unsatisfactory. In order to achieve the degree of flatness specified in many current construction projects an especially accurate and true straight-edge is again used on the concrete. The history of this highly accurate straight-edge may be traced to its use in leveling roadway surfaces during highway construction. For this reason the name "highway straight-edge" is most often used to describe such a tool.
Used as a hand tool, a highway straight-edge can commonly range in length from 6 to 16 feet (1.8 to 4.8M) and can be handled by one man. In some situations it is utilized in the initial screeding process as a standard straight-edge with workmen stationed at each end. In primary applications, a long handle is attached to the center of the beam to be used by a single workman. A special mechanical device is used at the point where the handle attaches to the beam. This mechanism allows the user to readily adjust the angle of the straight-edge beam, normal to the surface being leveled, as it is pulled and pushed over the wet concrete. This angle adjustment is beneficial in allowing the straight-edge to remove the high and low areas caused by bull floating without affecting the smooth finish created by that process.
A highway straight-edge beam can be made from a variety of materials. However, due to the nature of usage, including contact with wet concrete, each type of material utilized for a highway straight-edge exhibits its own advantages and disadvantages in terms of effectiveness, durability, weight, cost and service life.
The evolution of the highway straight-edge has led to the use of extruded magnesium tubing of rectangular cross section to act as a beam. Low weight and relative rigidity makes this material suitable for the application.
A problem arises however as the straight-edge tube experiences normal wear due to abrasion from contact with the concrete. This normal wearing away of material from the contact face brings about an asymmetric cross section of material about the original neutral axis of the tube. The extruded magnesium tube contains balanced internal stresses which is inherent of the extrusion manufacturing process. As the cross section of the beam becomes progressively asymmetric due to wear, the internal stresses also become unbalanced and the straight-edge tube becomes curved or bowed. This bowing or deflection occurs relatively soon in the life of the straight-edge and is most apparent at the center of longer tubes.
A deflected or bowed straight-edge must be straightened if it is to continue useful service. Presently no accurate method of straightening is available and replacement is costly.
One common straightening procedure is to support the straight-edge at its ends with the bowed center arching upward. Then a heavy workman attempts to use his weight by gravity at the center to bend the tube straight, often with unsatisfactory results. Typically either a string line is stretched from end to end or a visual inspection along the length of the tube is used to gauge the degree of straightness correction achieved.
Specifications for concrete floors in many current construction projects require ever higher degrees of flatness. A ready means of detecting undesirable deflection of a highway straight-edge, in addition to providing for its correction through controlled adjustment, is not found in the prior art.